1. Field of the Invention
The present invention relates to oscillator circuits, such as voltage controlled oscillator (VCO) circuits, and in particular, oscillator circuits having automatic level control for controlling the amplitude of the oscillator output signal so as to control the amount of phase noise within such signal.
2. Description of the Related Art
In oscillator circuit design, particularly when designing VCO circuits for use in a phase lock loop (PLL), a key design parameter is that of the phase noise performance of the oscillator. As is well known in the art, the term phase noise is generally used for. describing short term, random frequency fluctuations of a signal, such as that generated by an oscillator. The frequency stability of the oscillator circuit is a measure of the degree to which the oscillator maintains the same value of output signal frequency over a given interval of time. The ideal output signal from a sine wave oscillator is generally described as:
V(t)=Vo* sin(2xcfx80ft)
where: V(t)=oscillator output signal
Vo=nominal amplitude of oscillator output signal
f=frequency in Hertz (Hz)
t=time in seconds
while the instantaneous output signal of the oscillator is generally represented by:
V(t)=Vo*{1+A(t)}*sin{2xcfx80ft+xcex94xcfx86(t)}
where: A(t)=normalized amplitude fluctuations of oscillator output signal
xcex94xcfx86(t)=phase fluctuations of oscillator output signal in radians
Amplitude fluctuations in an oscillator signal can be removed, at least partially, by using a limiting amplifier. However, the phase noise of such a signal cannot be filtered out by any conventional means, and must be minimized at the point of generation. The magnitude of an oscillator""s phase noise close in to the carrier can be expressed by Leeson""s equation, as follows:
L(xcex94xcfx89)=10*log[2FkT/Psig*(xcfx89o/2Qxcex94xcfx89)2]
where: L(xcex94xcfx89)=phase noise power spectral density of the oscillator in dBc/Hz
F=equivalent noise factor of the negative R cell
Psig=signal power in the oscillator
xcfx89o=center frequency of the oscillator
Q=loaded Q (quality factor) of the oscillator tank
xcex94xcfx89=frequency offset from the center frequency
k=Boltzmann""s constant
T=temperature in Kelvin
As predicted by Leeson""s equation, phase noise can be reduced by increasing oscillation amplitude. Since Psig is proportional to Vo2, the phase noise decreases by a factor of 4 each time the oscillation voltage is doubled. However, most oscillators operate in a state of saturation. As a result, simply increasing the output signal level will cause adverse loading effects, such as those from increased base currents flowing through the bipolar transistors used for generating the output signal. These loading effects manifest themselves as a dramatic reduction in loaded Q of the tank circuit, degrading phase noise per the equation above.
The phase noise of the oscillator is often one of the more significant limiting factors in the performance capabilities of the host system, such as the ability of a radio receiver to reject undesired signals and preserve modulation fidelity of the frequency down converted signals. For example, in a radio transceiver, poor oscillator phase noise can lead to undesirable noise transmissions outside the desired bandwidth of the channel being transmitted. Hence, the phase noise of the oscillator circuit is one of the primary figures of merit used to determine the performance of the overall system.
Several factors play a role in the phase noise performance of the oscillator. The quality factor (xe2x80x9cQxe2x80x9d) of the resonator, the noise factor of the negative impedance cell and the oscillation signal amplitude all affect the phase noise of the oscillator, as shown in Leeson""s equation.
One conventional technique that has been used to address the effect of the oscillation signal amplitude upon the phase noise performance is to introduce automatic level control for establishing the amplitude of the oscillation signal at the appropriate level needed for achieving the desired phase noise. Such conventional automatic level control techniques have involved the detection of the oscillation signal (e.g., in terms of peak, peak-to-peak or RMS voltage) and comparing such detected voltage to a fixed voltage reference.
However, such automatic level control techniques do not allow the circuit to maintain the desired amount of control over the phase noise throughout significant variations in circuit operating temperature, power supply voltages and fabrication processes used to produce such circuit. Accordingly, it would be desirable to have a form of automatic level control for an oscillator circuit to optimize the amplitude performance of the oscillator in such a manner as to provide consistent control over its phase noise performance, and to maintain such control not withstanding variations in circuit operating temperature, power supply voltages and fabrication processes.
An oscillator in accordance with the presently claimed invention generates an oscillation signal having an amplitude that is automatically controlled for a selectively minimized phase noise. Automatic level control is used for controlling the amplitude of the oscillation signal such that the phase noise of the oscillation signal can be minimized or maintained at some selected level.
In accordance with one embodiment of the presently claimed invention, an oscillator with an oscillation signal amplitude that is automatically controlled for a selectively minimized oscillation signal phase noise includes oscillator circuitry and voltage control circuitry. The oscillator circuitry including first and second circuit terminals and an active circuit portion coupled between the first and second circuit terminals, responsive to an amplitude control signal, generates an oscillator signal, wherein the first and second circuit terminals are at first and second DC bias potentials, respectively, and the first DC bias potential is higher than the second DC bias potential. The voltage control circuitry, coupled to the oscillator circuitry and responsive to a comparison of a minimum signal voltage at the first circuit terminal and a maximum signal voltage at the second circuit terminal, generates the amplitude control signal such that a difference between the minimum and maximum signal voltages is maintained at a predetermined voltage amplitude.
In accordance with another embodiment of the presently claimed invention, a method for generating an oscillation signal amplitude that is automatically controlled for a selected oscillation signal phase noise includes the steps of
operating an active circuit between first and second circuit terminals, wherein the first and second circuit terminals are at first and second DC bias potentials, respectively, and the first DC bias potential is higher than the second DC bias potential;
receiving an amplitude control signal;
generating, in response to the amplitude control signal, an oscillator signal with a minimum signal voltage at the first circuit terminal and a maximum signal voltage at the second circuit terminal, wherein a difference between the minimum and maximum signal voltages is maintained at a predetermined voltage amplitude;
comparing the minimum and maximum signal voltages; and
generating the amplitude control signal in response to the voltage comparison.